Method for preparation of a silicone resin

Abstract: Disclosed is a method for the preparation of high molecular-weight silicone resins that have excellent storage stability in solution form. The method comprises the hydrolysis and condensation of a disiloxane and an alkyl silicate in an aqueous solution which contains at least 30 weight% alcohol and at least 5 weight% inorganic acid followed by isolation of the resin with an organic solvent which is poorly soluble in water and which has a dielectric constant of at least 4.

Description: The present invention relates to a method for the preparation of a silicone resin composed of monofunctional siloxane units (M units) and tetrafunctional siloxane units (Q units) which has a high molecular-weight and an excellent storage stability. Silicone resins composed of monofunctional (M) and tetrafunctional (Q) siloxane units are used as starting materials for pressure-sensitive adhesives, as reinforcing components for silicone rubber compositions, and as starting materials for film-forming materials such as silicone varnishes. Methods proposed for the preparation of there resins include: (a) cohydrolysis by the addition of water to a mixture of alkyl silicate and hydrolyzable trialkylsilane (US-A 2,857,356), and (b) dripping alkyl silicate into a mixed system of organosilicon compound(s) (selected from disiloxanes and hydrolyzable triorganosilanes) in aqueous hydrochloric acid solution (Japanese Patent Publication [Kokai] 3-60851 [60,851/1991]). To be useful as a film-forming material, a silicone resin should have a high molecular-weight. However, when high molecular-weight silicone resin is prepared by the methods taught in the above documents either the product undergoes gelation or it has a very poor storage stability in solution form. The present invention relates to a method for preparing a high molecular-weight silicone resin comprising: (I) hydrolizing and condensing (A) a disiloxane with the general formula [R 1 2 HSi] 2 O and (B) an alkyl silicate with the general formula Si(OR 2 ) 4 in (C) an aqueous solution which contains at least 30 weight percent alcohol and at least 5 weight percent inorganic acid, the percentages of both the alcohol and the inorganic acid being based on the total of said aqueous solution; wherein R 1 is a monovalent hydrocarbon group and R 2 is an alkyl group, the components (A) and (B) being used in a proportion such that the moles of component (A) / moles of component (B) is in the range of 0.05 to 0.6; and (II) isolating the silicone resin product with (D) an organic solvent which forms two layers with water and which has a dielectric constant of at least 4. The method of the present invention produces a high molecular-weight silicone resin that has excellent storage stability in solution form. The process comprises the hydrolysis and condensation of (A) a disiloxane and (B) an alkyl silicate in (C) an aqueous solution that contains an alcohol and an inorganic acid followed by isolation of the resin with (D) an organic solvent. Component (A) in the present invention is the source of the monofunctional siloxane unit (M unit) in the silicone resin product. This disiloxane has the following general formula: [R 1 2 HSi] 2 O R 1 in this formula is a monovalent hydrocarbon group. It is exemplified by alkyl groups such as methyl, ethyl, propyl, butyl and pentyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; and aralkyl groups such as benzyl and phenethyl. The methyl group is preferred. The disiloxane comprising component (A) can be 1,1,3,3-tetramethyldisiloxane, 1,3-dimethyl-1,3-diethyldisiloxane, 1,1-dimethyldisiloxane, 1,1,3,3-tetraphenyldisiloxane, 1,3-dimethyl-1,3-diphenyldisiloxane and mixtures of two or more of the preceding. Component (B) is the source of the tetrafunctional siloxane unit (Q unit) in the silicone resin product. This alkyl silicate has the following general formula: Si(OR 2 ) 4 R 2 in the preceding formula represents alkyl groups. They are exemplified by methyl, ethyl, and propyl. The methyl group is preferred due to ease of acquisition and because the hydrolysis rate of component (B) increases as the number of carbons in the alkyl group declines. The alkyl silicates of component (B) are available commercially as orthosilicates. Component (B) can be selected from methyl orthosilicate, ethyl orthosilicate, and isopropyl orthosilicate. Combinations of two or more of the preceding can also be used. Methyl orthosilicate and ethyl orthosilicate are preferred due to ease of acquisition and because the hydrolysis rate increases with diminishing number of carbons in the alkyl group in the alkyl silicate. Components (A) and (B) must be mixed in a proportion such that the moles of component (A)/moles of component (B) = 0.05 - 0.6. When this ratio of (A)/(B) falls below 0.05, high molecular weight silicone resin can be obtained but gelation also readily occurs. While gelation of the silicone resin product is inhibited at ratios in excess of 0.6, the silicone resin obtained will have a low molecular weight and poor film-forming properties. The preparative method of the present invention begins with the cohydrolysis/condensation of the aforementioned components (A) and (B) in aqueous solution (component (C) which contains at least 30 wt% alcohol and at least 5 wt% inorganic acid. Through the use of an aqueous alcohol solution for cohydrolysis, the preparative method of the present invention moderates the hydrolysis and condensation rate of component (B) and thereby prevents an increase in molecular weight of the silicone resin product to the point of gelation. There are no particular restrictions on the alcohols useable in the aqueous solution comprising component (C). Operable alcohols are methyl alcohol, ethyl alcohol, isopropyl alcohol, and n-propyl alcohol. Methyl alcohol and ethyl alcohol are preferred because lower boiling points facilitate post-synthesis solvent replacement by component (D) of the silicone resin. Component (C) must contain at least 30 wt% alcohol. An alcohol content of less than 30 wt% fails to moderate the hydrolysis and condensation rate of component (B) during the reaction between components (A) and (B) and, therefore, cannot prevent an increase in molecular weight by the silicone resin product to the point of gelation. In addition to alcohol, the aqueous solution comprising component (C) may also contain water-soluble organic solvents insofar as the object of the present invention is not impaired. There are no specific restrictions on the inorganic acid used in the aqueous solution comprising component (C). The inorganic acid is exemplified by hydrochloric acid, sulfuric acid, and nitric acid. Hydrochloric acid is preferred because it can be easily removed from the silicone resin reaction mixture after synthesis of the silicone resin. Component (C) must contain at least 5 wt% inorganic acid. An inorganic acid content below 5 wt% results in a slow component (A) cleavage during the reaction between components (A) and (B). This results in the preferential development of the hydrolysis and condensation of component (B) and, thus, an increase in molecular weight to the point of gelation. The quantity of component (C) used in the method of the present invention is not specifically restricted. Component (C) is preferably added in a quantity that makes possible a thorough hydrolysis and condensation of component (B). The reaction temperature is also not specifically restricted in the present invention, but the reaction temperature preferably does not exceed 30°C and more preferably does not exceed 15°C. A characteristic feature of the present invention is that the silicone resin afforded by the hydrolysis and condensation reaction is separated using organic solvent (component (D)) which is poorly soluble in water and which has a dielectric constant of at least 4. The basis for this feature is that the presence of inorganic acid in the silicone resin product results in a deterioration of the storage stability of the silicone resin and causes corrosion of treated surfaces when the silicone resin is employed as a coating agent. Inorganic acid impurities can be removed from the silicone resin by thoroughly dissolving the silicone resin from the reaction mixture into component (D) and washing with water. The organic solvent comprising component (D) must be poorly soluble in water, i.e., it must form two layers with water. It also must have a dielectric constant of at least 4, but it is not otherwise particularly restricted. The organic solvent comprising component (D) is specifically exemplified by ether solvents, such as diethyl ether (dielectric constant = 4.197), diisopropyl ether (dielectric constant = 4.49), anisole (dielectric constant = 4.33), phenetole (dielectric constant = 4.22), 1,2-dimethoxyethane (dielectric constant = 5.50), and 1,2-diethoxyethane (dielectric constant = 5.10); ketone solvents, such as methyl isobutyl ketone (dielectric constant = 13.11), and 2-heptanone (dielectric constant = 9.77); ester solvents, such as butyl acetate (dielectric constant = 5.01), isobutyl acetate (dielectric constant = 5.29), and diethyl sebacate (dielectric constant = 5.00); and other organic solvents, such as chloroform (dielectric constant = 4.335). The sequence for addition of component (D) is not specifically restricted. For example, a mixture of components (C) and (D) may first be prepared, and components (A) and (B) then reacted in such a system. Alternatively, components (A) and (B) may first be reacted in component (C), and component (D) then immediately added to the resulting system. The thorough dissolution from the reaction system of the high molecular-weight silicone resin product is made possible by the fact that the organic solvent comprising component (D) has a dielectric constant of at least 4 and is, therefore, highly polar. Furthermore, in the case of silicone resin solutions prepared using this solvent, drying of the solution affords an additional improvement in the solution storage stability. The quantity of component (D) is not specifically restricted in the preparative method of the present invention. Component (D) is preferably added in a quantity that can thoroughly dissolve the silicone resin that is produced. There are no specific restrictions on the molecular weight of the silicone resin afforded by the preparative method of the present invention. However, in order to make possible use of the silicone resin product as the base component for coatings, the silicone resin product preferably is a high molecular-weight silicone resin with a number-average molecular-weight of at least 3,000. The high molecular weight and excellent film-forming properties of the silicone resin afforded by the present invention enable its application as a silicone resin coating, as an additive for silicone rubber compositions, and as an additive for silicone varnishes. The present invention is explained in greater detail below by working examples. Example 1 20 g methanol, 12.1 g 7.4 N hydrochloric acid and 4.69 g 1,1,3,3-tetramethyldisiloxane were weighed into a 200 mL four-neck flask equipped with a spiral condenser, thermometer, and straight tube-equipped addition funnel. The solution temperature was reduced to ≤ 5°C by ice cooling, and nitrogen was passed through the system at a very low flow rate. While stirring the solution under these conditions, 30.4 g methyl orthosilicate was dripped in over 40 minutes from the straight tube-equipped addition funnel. The solution temperature rose to 13°C during this period due to the evolution of heat, but declined to 8°C by the time addition was completed. After addition, the reaction was stirred for an additional 15 minutes while cooling with ice. It was then stirred for 4 hours at room temperature. The reaction solution was colorless and transparent. 100 mL methyl isobutyl ketone and then 100 mL water were added to the reaction solution, whereupon phase-separation occured into two layers. The lower layer was taken off and shaken out with 50 mL of additional methyl isobutyl ketone. After phase-separatio, the upper layer was collected and combined with the water-containing upper layer from the reaction solution. 200 mL water was added to this and the system was shaken. Because this yielded an emulsion, 200 mL diethyl ether was added. Quiescence then resulted in a further phase-separation. The lower layer was discarded and the upper layer was collected and held for 10 hours over magnesium sulfate. The magnesium sulfate was then filtered off and the filtrate was concentrated to a solids concentration of 30 wt%. This concentrate was directly diluted with tetrahydrofuran to prepare a solution with a solids concentration of 0.2 wt%. This solution was subjected to gel permeation chromatography (GPC) using tetrahydrofuran as carrier solvent, and the dissolved component was thereby determined to have a number-average molecular-weight of 3,620, a weight-average molecular-weight of 7,050, and a dispersity of 1.95 (Mw/Mn). The dissolved component was determined by 29 Si nuclear magnetic resonance spectroscopic analysis (NMR) to be silicone resin with the following structural formula: [H(CH 3 ) 2 SiO 1/2 ] 0.35 [SiO 4/2 ] 1.0 . The concentrate was dripped onto a silicon wafer and the solvent was evaporated by standing in air at room temperature to produce a film with a thickness of approximately 1 micrometer. This film was subjected to transmission mode-based structural analysis using a Fourier-transform infrared spectrochemical analyzer (FTIR). A broad, strong absorption peak assigned to the siloxane bond was observed in the region of 1100 cm -1 , a sharp, strong absorption peak assigned to the Si-CH 3 group was observed in the region of 1260 cm -1 a sharp, strong absorption peak assigned to the SiH group was observed in the region of 2150 cm -1 , and a sharp, medium absorption peak assigned to the C-H group was observed in the region of 2960 cm -1 . Neither the viscosity of the solution nor the molecular weight of the dissolved component underwent variation during storage of the concentrate at room temperature for 1 month sealed in a wide-mouth high-density polyethylene bottle. Example 2 20 g methanol, 12.1 g 7.4 N hydrochloric acid, and 6.70 g 1,1,3,3-tetramethyldisiloxane were weighed into a 200 mL four-neck flask equipped with a spiral condenser, thermometer, and straight tube-equipped addition funnel. The solution temperature was reduced to ≤ 5°C by ice cooling, and nitrogen was passed through the system at a very low flow rate. While stirring the solution under these conditions, 30.4 g methyl orthosilicate was dripped in over 40 minutes from the straight tube-equipped addition funnel. The solution temperature rose to 14°C during this period due to the evolution of heat, but declined to 8°C by the time addition was completed. After addition, the reaction was stirred for an additional 15 minutes while cooling with ice. It was then stirred for 4 hours at room temperature. The reaction solution was colorless and transparent. 100 mL methyl isobutyl ketone and then 100 mL water were added to the reaction solution, whereupon phase-separation occurred into two layers. The lower layer was taken off and shaken out with 50 more mL of methyl isobutyl ketone. After phase-separation, the upper layer was collected and combined with the water-containing upper layer from the reaction solution. 200 mL water was added to this and the system was shaken. Because this yielded an emulsion, 100 mL diethyl ether was added. Quiescence then resulted in a further phase-separation. The lower layer was removed, another 200 mL water was added, and the system was shaken. A rapid phase-separation occurred upon standing in this case. The upper layer was collected and held for 10 hours over magnesium sulfate. The magnesium sulfate was subsequently filtered off and the filtrate was concentrated to a solids concentration of 30 wt%. This concentrate was directly diluted with tetrahydrofuran to prepare a solution with a solids concentration of 0.2 wt%. This solution was subjected to GPC, and the dissolved component was thereby determined to have a number-average molecular-weight of 3,310, a weight-average molecular-weight of 5,760, and a dispersity of 1.74 (Mw/Mn). The dissolved component was determined by NMR analysis to be silicone resin with the following structural formula: [H(CH 3 ) 2 SiO 1/2 ] 0.5 [SiO 4/2 ] 1.0 . The concentrate was dripped onto a silicon wafer and the solvent was evaporated by standing in air at room temperature to produce a film with a thickness of approximately 1 micrometer. This film was subjected to transmission mode-based structural analysis using a FTIR analyzer. A broad, strong absorption peak assigned to the siloxane bond was observed in the region of 1100 cm -1 , a sharp, strong absorption peak assigned to the Si-CH 3 group was observed in the region of 1260 cm -1 , a sharp, strong absorption peak assigned to the SiH group was observed in the region of 2150 cm -1 , and a sharp, medium absorption peak assigned to the C-H group was observed in the region of 2960 cm -1 . Neither the viscosity of the solution nor the molecular weight of the dissolved component underwent variation during storage of the concentrate at room temperature for 1 month sealed in a wide-mouth high-density polyethylene bottle. Comparison Example 1 5 g methanol, 20 g toluene, 12.1 g 7.4 N hydrochloric acid, and 4.69 g 1,1,3,3-tetramethyldisiloxane were weighed into a 200 mL four-neck flask equipped with a spiral condenser, thermometer, and straight tube-equipped addition funnel. The solution temperature was reduced to ≤ 5°C by ice cooling, and nitrogen was passed through the system at a very low flow rate. While stirring the solution under these conditions, 30.4 g methyl orthosilicate was dripped in over 40 minutes from the straight tube-equipped addition funnel. The solution temperature rose to 13°C during this period due to the evolution of heat, but declined to 9°C by the time addition was completed. After addition, the reaction was stirred for an additional 15 minutes while cooling with ice. It was then stirred for 4 hours at room temperature. The reaction solution was an emulsion at this point. When the system was subsequently held at room temperature while purging with nitrogen, the reaction solution gelled within 12 hours.

Claims: A method for preparing a high molecular-weight silicone resin comprising: (I) hydrolizing and condensing (A) a disiloxane with the general formula [R 1 2 HSi] 2 O and (B) an alkyl silicate with the general formula Si(OR 2 ) 4 in (C) an aqueous solution which contains at least 30 weight percent alcohol and at least 5 weight percent inorganic acid, the percentages of both the alcohol and the inorganic acid being based on the total of said aqueous solution; wherein R 1 is a monovalent hydrocarbon group and R 2 is an alkyl group, the components (A) and (B) being used in a proportion such that the moles of component (A) / moles of component (B) is in the range of 0.05 to 0.6; and (II) isolating the silicone resin product with (D) an organic solvent which forms two layers with water and which has a dielectric constant of at least 4. The method of claim 1, wherein R 1 is a monovalent hydrocarbon group selected from alkyl groups, aryl groups and aralkyl groups and/or R 2 is an alkyl group selected from methyl, ethyl, propyl and isopropyl. The method of claim 1 or 2, wherein the disiloxane is selected from 1,1,3,3-tetramethyldisiloxane, 1,3-dimethyl1,3-diethyldisiloxane, 1,1-dimethyldisiloxane, 1,1,3,3-tetraphenyldisiloxane and 1,3-dimethyl-1,3-diphenyldisiloxane. The method of any of claims 1 to 3 wherein the alcohol is selected from methyl alcohol, ethyl alcohol, iso-propyl alcohol and n-propyl alcohol. The method of any of claims 1 to 4 wherein the aqueous solution (C) also contains a water-soluble organic solvent. The method of any of claims 1 to 5 wherein the inorganic acid is selected from hydrochloric acid, sulfuric acid and nitric acid. The method of any of claims 1 to 6 wherein the hydrolysis and the condensation occur at temperatures of 30°C or less. The method of any of claims 1 to 7 wherein the organic solvent (C) is selected from ether solvents, ketone solvents, ester solvents and chloroform.

Inventors: MINE KATSUTOSHI, NAKAMURA TAKASHI, SASAKI MOTOSHI, MINE, KATSUTOSHI, NAKAMURA, TAKASHI, SASAKI, MOTOSHI, MINE, KATSUTOSHI, C/O DOW CORNING TORAY SILIC. CO, NAKAMURA, TAKASHI, C/O DOW CORNING TORAY SILC. CO, SASAKI, MOTOSHI C/O DOW CORNING TORAY SILIC. CO
Applicants: DOW CORNING TORAY SILICONE, DOW CORNING TORAY SILICONE COMPANY, LIMITED
Application year: 1994
Main classification: C08G77
IPC codes: C08G77